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Research Papers

Design of a Tri-Axial Force Measurement Transducer for Plantar Force Measurements

[+] Author and Article Information
Li Weizi

Jiao Tong University Joint Institute,
University of Michigan-Shanghai,
Shanghai 200240, China
e-mail: patronum1991@sjtu.edu.cn

Dugnani Roberto

Jiao Tong University Joint Institute,
University of Michigan-Shanghai,
Shanghai 200240, China
e-mail: roberto.dugnani@sjtu.edu.cn

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT,AND CONTROL. Manuscript received May 21, 2017; final manuscript received January 18, 2018; published online March 13, 2018. Assoc. Editor: Soo Jeon.

J. Dyn. Sys., Meas., Control 140(8), 081012 (Mar 13, 2018) (10 pages) Paper No: DS-17-1263; doi: 10.1115/1.4039205 History: Received May 21, 2017; Revised January 18, 2018

Transducers for spatial plantar force measurements have numerous applications in biomechanics, rehabilitation medicine, and gait analysis. In this work, the design of a novel, tri-axial transducer for plantar force measurements was presented. The proposed design could resolve both the normal and the shear forces applied at the foot's sole. The novelty of the design consisted in using a rotating bump to translate the external loads into axial compressive forces which could be measured effectively by conventional pressure sensors. For the prototype presented, multilayer polydimethylsiloxane (PDMS) thin-film capacitive stacks were manufactured and used as sensing units, although in principle the design could be extended to various types of sensors. A quasi-static analytic solution to describe the behavior of the transducer was also derived and used to optimize the design. To characterize the performance of the transducer, a 3 cm diameter, 1 cm tall prototype was manufactured and tested under various combination of shear and normal loading scenarios. The tests confirmed the ability of the transducer to generate strong capacitive signals and measure both the magnitude and direction of the normal and shear loads in the dynamic range of interest.

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Figures

Grahic Jump Location
Fig. 1

Three-dimensional (3D) rendering of the designed transducer with major components highlighted. Two of the three sensing stacks (hidden) are shown by dotted lines. In the top right corner, the transducer is shown with its protective cover in place.

Grahic Jump Location
Fig. 2

Schematic view of the analyzed transducer including the key dimensions and location of both external and reaction loads. The suffices 1, 2, and 3 refer to capacitance stacks 1, 2, and 3, respectively.

Grahic Jump Location
Fig. 3

Schematic representation of displacements at the perimeter of the bump, z(θ), and at the center of each transducer stack: u1, u2, and u3

Grahic Jump Location
Fig. 4

Schematic representation of the multilayer structure and assembly of the capacitor stack units (left). Image of capacitor stack during the assembling stage (top right). In the lower part of the figure, the assembly configuration of the three capacitor stacks with the terminals 120 deg apart is also shown schematically.

Grahic Jump Location
Fig. 5

Illustration of two-dimensional analytical model including equivalent horizontal and vertical stiffness for the capacitor stacks and flexure beams

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Fig. 6

Detailed sketch of the curved, flexure beams including some relevant dimension

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Fig. 7

Loading wedge and alignment pin used in the shear/tension test (left). Shear loading test setup for the transducer assembly.

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Fig. 8

(a) Compressive axial force versus recorded displacement for C1 capacitance stack (25 layers; loading frequencies 0.4 Hz and 0.8 Hz). (b) Corresponding compressive axial force versus capacitance for the same sensor and loading frequencies.

Grahic Jump Location
Fig. 9

Predicted forces and forces direction for the full transducer mechanical test (|T|/N = 0.2). Filled markers represent data corresponding to axial loads higher than 5 N. Open markers represent data for low axial loads (less than 5 N).

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